1. Field of the Invention
The present invention relates to a grating (diffraction grating) which is a wavelength separating/selecting element for use in spectroscope, optical spectrum analyzer, WDM monitor, wavelength separator, etc. and a method for producing the grating. More particularly, the present invention relates to a blazed holographic grating prepared by a holographic exposure method and a method for producing the blazed holographic grating.
2. Description of the Related Art
A grating (diffraction grating) is a wavelength separating/selecting element for use in spectroscope, wavelength separator, etc. Various gratings are known by the profile of section of groove. These gratings include one called “blazed holographic grating (BHG)”.
As a basic method for producing the blazed holographic grating there has been heretofore known one disclosed in Japanese Patent No. 1,046,763. In accordance with the production method disclosed in the above cited patent reference, a photoresist which has been spread over a substrate such as quartz and glass is exposed to interference band developed by two-beam interference, and then developed to form a resist pattern having a sinusoidal or half-sinusoidal section thereon (holographic exposure process). Thereafter, the substrate is irradiated with an ion beam obliquely with this resist pattern as a mask to form a desired braze angle of the substrate until the photoresist disappears so that it is etched to form a diffraction grating groove having a sawtooth section thereon. By coating the surface of the grating groove with a reflective material such as aluminum and gold to raise its reflectance, a blazed holographic grating allowing the concentration of energy of diffracted ray on a specific wavelength is obtained.
The grating thus prepared is greatly advantageous in that it has a high diffraction efficiency and generates little stray light but is also disadvantageous in that it takes much time to produce, adding to cost and the difference of performance among products can be difficultly eliminated. In an attempt to reduce the cost by mass production and eliminate the dispersion of performance, it has been practiced to produce a replica grating as a molded resin product from the original grating produced according to the aforementioned procedure as a matrix. In some detail, duplication of the original grating onto a desired substrate such as glass with a synthetic resin is conducted to prepare a negative mold from which a replica grating is then prepared. The replica grating thus prepared is somewhat inferior to the original grating in performance but is advantageous in that even when mass-produced, it has a high uniformity in performance, and it can be provided at drastically reduced cost. Of course, the performance of the original grating must be good to provide the replica grating thus prepared with a sufficient performance.
The basic procedure of engraving a grating groove directly on a substrate such as glass to prepare a blazed holographic grating has been described above. In practice, however, the performance of grating is impaired by various causes.
For example, the resist pattern (sinusoidal or half-sinusoidal diffraction grating groove) prepared by holographic exposure process has a surface roughness developed by the granularity of fine particles of photoresist and the noise generated during exposure. Since this resist pattern is obliquely etched to make sawtooth cutting while similarly cutting the substrate, the surface roughness of the photoresist appears on the reflective surface of the grating groove on the substrate. Further, end point control can be considerably difficultly made to terminate the etching just when the resist pattern formed on the substrate disappears during ion beam etching. Thus, in most cases, overetching tends to occur. Thus, the vertex of the grating groove is rounded, deteriorating the linearity of the two surfaces forming the vertex (i.e., giving a curved vertex). On the other hand, when the etching time is reduced to avoid such overetching, underetching can occur, leaving the desired vertex of the grating groove uncompleted.
FIG. 6 illustrates an AFM (atomic force microscope) image of the grating groove of a blazed holographic grating prepared by the aforementioned related art production method. As can be seen in FIG. 6, the reflective surface of the grating groove is roughened and the vertex of the grating groove is rounded and has an uneven surface. Further, the grating groove shows a crosswise meandering (periodical crosswise disturbance of groove). Such an improper profile of the grating groove makes it difficult to obtain a sufficient diffraction efficiency. Further, when this grating is used as a spectroscope, much stray light is generated because much scattered light is generated.